What is the Data Stamp Rule—and What are the Exceptions?

Sometimes firearms can safely shoot more than one caliber. Here’s how to tell which ones.  

by posted on September 20, 2023
Rao Data Stamp 357 Lede

Always make certain you have the proper ammunition that can safely be shot out of the firearm you will be using. To correctly match ammunition to your gun, you must look at the data stamp on the firearm. The data stamp is usually found on the barrel of a rifle, shotgun or revolver. On a semi-automatic pistol, the data stamp is usually found on the slide or on the rear of the barrel where the chamber is located, and visible through the breech opening cutout when the action is closed.

The golden rule is to always match up the ammunition with the data stamp on the firearm, and only shoot the ammunition designation on the head stamp to the data stamp on the firearm. As with most rules, there are exceptions. It is important that firearms instructors as well as recreational shooters know these exceptions to be safe while shooting ammunition that differs from the data stamp on the gun.

This is especially important for instructors to know because if there is an accident in their class, it is ultimately their responsibility. Exceptions to the data stamp rule can be found in revolvers, semi-automatic pistols and in rifles.

The most common caliber exception to the data stamp rule is the .357 S&W Magnum. When revolvers ruled the handgun world, it was common knowledge that you could shoot a .38 S&W Spl. out of a .357 revolver. Commonly known as the .38 Special, not to be confused with the obsolete .38 S&W cartridge. The .357 S&W and the .38 S&W Special both have a bullet diameter of .357 inch.

The .357 S&W Magnum cartridge has much higher pressure than the .38 S&W Special cartridge. Therefore, it is safe to shoot the types of .38 S&W Special ammunition out of a firearm chambered for .357 S&W Magnum. There are three .38 S&W Special cartridges: the standard .38 S&W Spl. cartridges; the .38 +P cartridges; and the .38 +P+ cartridges. The + and P designations indicate higher than normal pressure.

If you have a firearm chambered for .38 S&W Spl. you cannot assume that it is safe to shoot ammunition stamped +P or +P+ out of your revolver. The +P or +P+ designation is usually not stamped on the barrel of the revolver. This information can be found in the owner’s manual. The .38 +P+ cartridge is the highest pressure of the three. The next highest pressure is the +P cartridge, followed by the standard .38 S&W Special cartridge. Remember, you can go down in pressure and shoot lower-pressure ammunition out of a firearm, but you cannot shoot a cartridge with a higher pressure than for which the firearm is rated.

 The .44 Rem. Mag. cartridge has a lower pressure ammunition counterpart. The lower pressure counterpart of the .44 Rem. Mag. is the .44 S&W Spl. Both cartridges have a bullet diameter of .429 inches. It is safe to shoot a .44 S&W cartridge out of a firearm chambered for the .44 Rem. Mag. It is very dangerous to shoot any magnum cartridge out of a firearm only rated for a Special cartridge. Therefore, never shoot a .44 Rem. Mag. cartridge, which has higher pressure, out of a firearm chambered for .44 Special.

The .460 S&W Mag. has three “sister” cartridges. If you have a firearm chambered for .460 S&W Mag., depending on the gun, you can also shoot a .454 Casull cartridge or a .45 Colt cartridge out of this same revolver. Note: The .45 Colt is frequently mislabeled .45 “Long” Colt. The bullet diameter of all three bullets is .452 inches. The order of highest to lowest pressure is .460 S&W Magnum, .454 Casull, and the .45 Colt. Once again, you can shoot “down” pressure cartridges, meaning, you can shoot lower pressure cartridges out of a firearm that can accept higher pressure cartridges. Never shoot a higher-pressure cartridge out of a firearm that cannot accept those high pressures. For example, shooting a .460 S&W Magnum cartridge can have disastrous consequences if shot out of a revolver not designed for lower pressures such as a .45 Colt.

The .45 ACP and the .45 Automatic are the same cartridge. Both bullets have a diameter of .452 inches. The .45 Colt is not the same and is not interchangeable with the .45 ACP or the .45 Auto. The .45 ACP and the .45 Auto cartridge are rimless rounds, and the .45 Colt is a larger, rimmed cartridge.

The 9mm cartridge can be confusing. There are many interchangeable 9mm cartridges, depending on the firearm. It is very important to know which 9mm cartridges are interchangeable, so your firearm functions properly. The most common 9mm rounds are the 9mm Luger, 9mm Parabellum, 9mm NATO, and the 9X19. All these cartridge designations are interchangeable.

The 9mm Makarov, or 9X18 Makarov, is a Russian round and is NOT interchangeable with other firearms chambered in 9mm. This cartridge is a Soviet pistol and a Soviet submachine gun round. Occasionally, a student will show up to class with a 9mm Makarov pistol and 9mm Luger or 9mm Parabellum ammunition. 9mm Makarov ammunition is not as readily available as other 9mm rounds. When a student shows up with a 9mm Makarov pistol and different ammunition that the firearm is not designed to accept, they must then borrow someone else’s firearm to complete the class. As an NRA Certified Instructor, it is crucial to keep a close eye on all firearms and ammunition your students bring with them to the range portion of class. Make sure you inspect each firearm to ensure it is designed to accept the ammunition that it accompanies.

Another 9mm cartridge that can be confusing is the 9mm Kurz. The 9mm Kurz, .380 Auto, 9X17, 9mm Browning, 9mm Short, and 9mm Corto are all interchangeable cartridges. I encountered this in class several years ago and must admit, I was stumped. The student had a 9mm Kurz pistol and .380 Auto ammunition. She was a competitive shooter, and this was her firearm and ammunition of choice. After doing some research, and learning something new, the student continued with class and successfully completed training.

The .223 Rem. cartridge and the 5.56X45mm NATO is always a discussion in class. The .223 Rem. cartridge has a diameter of .224 inches (this sounds like a misprint, but it actually is a .224-inch diameter bullet) and the 5.56X45mm NATO has a diameter of .219 inches. Even though the 5.56X45mm NATO cartridge is slightly smaller, it has higher pressure than the .223 Rem. cartridge.

If you have either a bolt-action or a semi-automatic action in a firearm chambered for a 5.56X45mm NATO round, you should be able to safely shoot a .223 Rem. cartridge out of these guns. The .223 Rem. is another story though. Since the 5.56X45mm NATO has higher pressures than the .223 Rem., you need to be more cautious.

The .223 Rem. is usually found in either a bolt-action rifle or a semi-automatic rifle. You should avoid shooting a 5.56X45mm NATO cartridge out of a .223 Rem. semi-automatic rifle. If you have a bolt-action .223 Rem., you should be able to shoot a 5.56X45mm NATO cartridge out of it. This is because a bolt action is stronger than a semi-automatic action, so the bolt action should be able to handle the higher pressures. Of course, always consult your owner’s manuals to know what is safe to shoot out of your firearms.


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